Aircraft electric power systems and other power distribution systems are exposed to a wide range of disturbances, which may initiate transient electrical events. For example, equipment failure and lightning strikes can lead to current and voltage transients. Short-circuit conditions can also result in transient surges. Such transients may cause electrical arcing in the power distribution system and/or overheating of circuit components, which in turn may present fire hazards.
Against this background, there is a tendency to design aircraft and other electric power systems to operate at higher and higher voltages and hence lower currents, allowing reduction in the weight of the cables of the system. Both DC and AC power systems may be used in aircraft.
In the prior art, transients have traditionally been measured using a current transducer, which in DC systems must interrupt the circuit under measurement in order to be inserted therein. FIG. 1 shows such a prior art system, including a power distribution system 11 having a DC source 12 connected to a load 14. An arc event 15 is measured by a current transducer 13, which is connected in series in the circuit between the load 14 and the power supply 12. The arc produces a back emf in the circuit of approximately 20V, the back emf being substantially independent of the source voltage. As higher source voltages are used, the change in current associated with the arc event becomes increasingly small compared with the overall system current, and therefore the change in current is harder to measure.
An alternative prior art approach involves the measurement of a travelling wave superimposed on the system voltage, brought about by the electrical transient. This method relies on knowledge of a characteristic circuit impedance which is uncontrolled in an electrical installation.